Load balancing in wireless communication network

ABSTRACT

For load balancing in a wireless communication network comprising at least one subscriber terminal (T 1 , T 2 , T 2 ) and a plurality of access points (AP 1 , AP 2 , AP 3 ), a load control device ( 21; 110 ) is used which is located outside of said subscriber terminal, wherein said load control device is adapted to process information related to a load in said wireless communication network and to instruct roaming of said subscriber terminal from an associated access point to another one of said plurality of access points. Access point status information (APST) determined in said plurality of access points (S 10 ; S 110 ) is received and communication status information related to said plurality of access points (S 20 ; S 120 ) is determined. The subscriber terminal processes (S 20 ; S 120 ) these information into roaming support information (RSUP), which are in turn processed (S 40 ; S 160 ) in said load control device an access point related load based roaming analysis. On this basis, it is decided (S 50 ; S 170 ) by the load control device, whether said subscriber terminal is to be associated with another one of said plurality of access points.

FIELD OF THE INVENTION

The present invention relates to a method for load balancing in awireless communication network, a corresponding system, a load controldevice, a corresponding access point usable in a wireless communicationnetwork, a corresponding network element usable in a wirelesscommunication network, a corresponding subscriber terminal usable in awireless communication network and a computer program product forimplementing the method for load balancing in a wireless communicationnetwork.

BACKGROUND OF THE INVENTION

In the last years wireless communication networks, for example for dataand/or speech transmissions, became more and more important. One examplefor such a wireless communication network is a wireless local areanetwork (WLAN), independent of a radio frequency or the like (infraredetc.) used for this network. In such a WLAN, subscriber terminals, suchas personal computers, telecommunication equipments, mobile phones,personal digital assistants and the like, are able to performcommunications with each other or with corresponding subscriberterminals of other networks (for example wired LANs, WLANs, fixed ormobile telecommunication networks and the like) via correspondingcommunication protocols.

The general architecture of a WLAN is commonly known and thus describedonly shortly. The key elements of a WLAN are the subscriber terminalsand access points (AP) with which the subscriber terminal communicatesover a wireless communication interface, e.g. radio or infrared based.An access point covers a specific area, which is referred to hereinafteras a cell. The size of a cell may vary in dependence of the environment,network operator specifications, number of associated subscribers andthe like. The AP is adapted to control communications of the subscriberterminals within this cell, for example, by allocating frequencychannels, establishing connections for the subscriber terminals,forwarding data to a destination terminal and the like. A subscriberterminal is normally associated with one access point, which is referredto hereinafter as the serving AP. Furthermore, a distribution network isprovided to which the access points are connected. Via the distributionnetwork, communication connections between different APs or externalnetworks (e.g. fixed networks, mobile telecommunications networks suchas GSM, UMTS, and the like) can be established for a subscriberterminal. In the case of mobile subscriber terminals, there is a casethat a subscriber terminal leaves the cell of its serving AP. In thiscase, roaming is executed. Roaming means that the subscriber terminalsearches an available AP whose connection quality is better than apredetermined threshold or the like and switches the connection to thisother available AP, which becomes the serving AP. The decision whether aroaming is to be executed is based, for example, on signal strengthmeasurements and the like. WLANs are implemented according to specificstandards. One of these standards is, for example, the IEEE (Instituteof Electrical and Electronics Engineers) 802.11 standard or itsrespective extensions such as the IEEE 802.11h standard, which arecommonly known to persons skilled in the art.

In the IEEE 802.11 standards, in particular MAC (MAC: Medium AccessControl) and PHY (PHYsical layer) protocols are defined. MAC protocol isused, for example, to allow interoperability between compatible physicallayers, to reduce a collision probability between different subscriberterminals, and the like. Moreover, the IEEE 802.11 MAC protocol definesbeacon frames sent at regular intervals by the access point to allowstations to monitor the presence of the access point. The IEEE 802.11MAC protocol also gives a set of management frames including ProbeRequest frames which are sent by a subscriber terminal and are followedby Probe Response frames sent by an available access point, to allow asubscriber terminal to scan actively if there is an access pointoperating on a certain channel frequency and to show to the subscriberterminal what parameter settings this access point is using.Additionally, a MAC address is provided which is used as anidentification element for the respective WLAN elements.

According to the prior art, roaming of a subscriber terminal from one APto another AP is initialized on the basis of specific configurationsettings in the wireless communication network. In accordance with theseconfiguration settings, a subscriber terminal will decide on the basisof communication reception quality measurements whether the recentreception quality is sufficient or not. If not, a commonly known roamingprocedure will be initialized by the subscriber terminal.

However, according to this conventional roaming procedure, a loadsituation in the wireless communication network is not considered in thedecision for roaming. In other words, a subscriber terminal could beswitched or associated with an AP in which the load situation is suchthat the communication connection quality is not satisfying. Thus, inconventional wireless communication networks, the load is not balancedbetween the respective APs.

In document EP 1 156 623 A1, a wireless LAN is described in which theroaming procedure is added by a load balancing function. In order tobalance the load within the WLAN, the subscriber terminal receives loadinformation concerning the loading status of the access point fromaccess points. Then, the subscriber terminal may select a communicationconnection with one of the access points by using a cost function inwhich the received load information is considered.

However, in specific situations, there might arise a problem in that theload based roaming procedure is not performed in an advantageous manner.In a cellular type of environment the client might be “hesitant” toselect the correct cell on the basis of conventional measurementinformation. Thus, there might be a case that the subscriber terminalhas measurement information on hand which causes a very easy or frequentchange of the cell, i.e. of the serving AP. In particular in the middleof a transmission this causes undesired pauses to applications, e.g.data transmissions or the like. Typically the roaming in WLAN cellsrequires some time when changing from serving AP to the neighbor AP. Inother words, the effect of such easy roaming is a “stopped” applicationthat continues not until the signaling has been performed in the roamingstate. In particular when the subscriber terminal is located in adensely utilized wireless network environment (i.e. a plurality ofsubscriber terminals is communicating in the respective cells) therecould be a so-called “ping-pong” effect. In other words, the decision tochange the serving access for the subscriber terminal is made ratherfrequently and even such that the same two APs are involved in theroaming.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to achieve animproved load balancing in a wireless communication network.

This object is achieved, for example, by a method of load balancing in awireless communication network, said wireless communication networkcomprising at least one subscriber terminal adapted to establish andperform a wireless communication connection in said wirelesscommunication network, a plurality of access points adapted to controlsaid wireless communication connection of said at least one subscriberterminal and to exchange information with said at least one subscriberterminal, wherein one of said plurality of access points is associatedwith said at least one subscriber terminal, and a load control devicelocated outside of said subscriber terminal, said load control devicebeing adapted to process information related to a load in said wirelesscommunication network and to instruct roaming of said subscriberterminal from said associated one of said plurality of access points toanother one of said plurality of access points, said method comprisingthe steps of receiving, in said subscriber terminal, access point statusinformation determined in said plurality of access points, determiningcommunication status information related to said plurality of accesspoints, and processing said received access point status information andsaid communication status information in order to obtain roaming supportinformation, processing, in said load control device, said roamingsupport information by an access point related load based roaminganalysis and deciding, on the basis of a result of said access pointrelated load based roaming analysis, whether said subscriber terminal isto be associated with another one of said plurality of access points,and if so, initializing roaming of said subscriber terminal to saidanother one of said plurality of access points in said wirelesscommunication network.

Furthermore, this object is achieved, for example, by a system for loadbalancing in a wireless communication network, said wirelesscommunication network comprising at least one subscriber terminaladapted to establish and perform a wireless communication connection insaid wireless communication network, and a plurality of access pointsadapted to control said wireless communication connection of said atleast one subscriber terminal and to exchange information with said atleast one subscriber terminal, wherein one of said plurality of accesspoints is associated with said at least one subscriber terminal, said,system comprising access point load status monitoring means located ineach one of said plurality of access points and adapted to measure atraffic load of an access point and to transmit access point statusinformation, roaming support means located in said subscriber terminaland adapted to receive said access point status information from saidplurality of access points, to determine communication statusinformation related to said plurality of access points, to process saidreceived access point status information and said communication statusinformation in order to obtain roaming support information, and totransmit said roaming support information, and a load control devicelocated outside of said subscriber terminal, said load control devicebeing adapted to process said roaming support information by an accesspoint related load based roaming analysis, to decide, on the basis of aresult of said access point related load based roaming analysis, whethersaid subscriber terminal is to be associated with another one of saidplurality of access points, and to initialize roaming of said subscriberterminal from said associated one to said another one of said pluralityof access points in said wireless communication network.

Moreover, this object is achieved, for example, by a load control devicefor load balancing in a wireless communication network, said wirelesscommunication network comprising at least one subscriber terminaladapted to establish and perform a wireless communication connection insaid wireless communication network, and a plurality of access pointsadapted to control said wireless communication connection of said atleast one subscriber terminal and to exchange information with said atleast one subscriber terminal, wherein one of said plurality of accesspoints is associated with said at least one subscriber terminal, whereinsaid load control device is located outside of said subscriber terminaland adapted to process roaming support information, received from asubscriber terminal and derived by said subscriber terminal from accesspoint status information of said plurality of access points, by anaccess point related load based roaming analysis, to decide, on thebasis of a result of said access point related load based roaminganalysis, whether said subscriber terminal is to be associated withanother one of said plurality of access points, and to initializeroaming of said subscriber terminal from said associated one to saidanother one of said plurality of access points in said wirelesscommunication network.

Furthermore, this object is achieved, for example, by an access pointusable in a wireless communication network, said wireless communicationnetwork comprising at least one subscriber terminal adapted to establishand perform a wireless communication connection in said wirelesscommunication network and further access points, said access point beingadapted to control said wireless communication connection of said atleast one subscriber terminal and to exchange information with said atleast one subscriber terminal, wherein said access point comprisesaccess point load status monitoring means adapted to measure a trafficload of an access point and to transmit access point status information,and a load control device as defined above.

Moreover, this object is achieved, for example, by a network elementusable in a wireless communication network, said wireless communicationnetwork comprising at least one subscriber terminal adapted to establishand perform a wireless communication connection in said wirelesscommunication network, and a plurality of access points adapted tocontrol said wireless communication connection of said at least onesubscriber terminal and to exchange information with said at least onesubscriber terminal, wherein one of said plurality of access points isassociated with said at least one subscriber terminal, wherein saidnetwork element is separated from and connected to said plurality ofaccess points and comprises a load control device as defined above.

Furthermore, this object is achieved, for example, by a subscriberterminal usable in a wireless communication network, said wirelesscommunication network comprising a plurality of access points adapted tocontrol a wireless communication connection of said subscriber terminaland to exchange information with said subscriber terminal, wherein oneof said plurality of access points is associated with said subscriberterminal, said subscriber terminal comprising roaming support meansadapted to receive access point status information from said pluralityof access points, to determine communication status information relatedto said plurality of access points, to process said received accesspoint status information and said communication status information inorder to obtain roaming support information, and to transmit saidroaming support information to a load control device as defined above,wherein said subscriber terminal performs, in response to an instructionfrom said load control device, roaming from said associated one toanother one of said plurality of access points in said wirelesscommunication network, said another one of said plurality of accesspoints is indicated in said instruction from said load control device.

Moreover, this object is achieved, for example, by a computer programproduct usable for a data processing apparatus, comprising software codeportions for performing the steps of the method of load balancingdefined above when said product is run on said data processingapparatus.

Advantageous further developments of the present invention are as setout in the respective dependent claims.

The present invention is particularly useful to balance the loadsituation in wireless communication networks in such a manner that asubscriber terminal may perform a roaming to another AP in order toachieve a better data throughput even if the signal strength situationwould not require such a change of the serving AP. Thus, the capacity ofAPs available for the subscriber terminal is used in an improved manner.Furthermore, a so-called ping-pong effect during a roaming procedure fora subscriber terminal can be suppressed.

Load balancing according to the present invention is useful, forexample, in a WLAN environment, e.g. according to the IEEE 802.11standard and its extensions, such as IEEE 802.11h.

The access point status information may comprise an access pointidentification element, such as a MAC address and an access point loadstatus indicator, which is determined for each access point and definesa load situation for the access point.

Communication status information determined, for example, in thesubscriber terminal may comprise a received signal strength indicatorRSSI indicating the received signal strength for communications betweenaccess points and said subscriber terminal. Furthermore, a carrier tointerference ratio C/I per each access point may be determined for thecommunication status information. Additionally, a terminal transmitpower status may be determined for the communication status information.

The roaming support information may be obtained in the subscriberterminal by processing the received access point status information andthe communication status information. It may comprise statistics (tablesor lists) of access point related communication status and loadinformation derived or measured from the received access point statusinformation, such as RSSI statistics, C/I statistics, load statusinformation, AP/client transmit power status statistics and the like,which are allocated to the respective access points by the an accesspoint identification element.

The roaming support information may be processed in the access pointrelated load based roaming analysis by using a hand-off algorithm.Thereby, load and connection quality situations are calculated for theconnections between the subscriber terminal and the available accesspoints on the basis of the roaming support information. Thus, an optimalaccess point for being associated with the subscriber terminal can bedetermined on the basis of the respective load and connection qualitysituations.

Furthermore, processing parameters used in the access point related loadbased roaming analysis, for example in the hand-off algorithm, which arederived from the roaming support information, may be differentlyweighted. For example, parameters associated with a connection qualitysituation are provided with a higher priority than parameters associatedwith the load situation. Thus, operator specific settings for roamingand load balancing are further improved.

The load control functionality may be provided in different sites. Forexample, the access points may comprise the load control functionality.Alternatively, the load control may be located in a specific networkelement separated from the access points, wherein this network elementis connected with the access points in the wireless communicationnetwork.

As a further refinement, access point internal monitoring informationdetermined in the access points may be used for the load control. Theload control functionality determines, for example, access points whichare available for said subscriber terminal, e.g. by comparing the accesspoint identification elements in the roaming support information, andselects only the access point internal monitoring information of suchavailable access points. Then the load control processes the roamingsupport information and the selected access point internal monitoringinformation by means of an enhanced access point related load basedroaming analysis. On the basis of a result of the enhanced access pointrelated load based roaming analysis, it is decided whether thesubscriber terminal is to be associated with another access point. Ifthis is the case roaming of the subscriber terminal to this other accesspoint in said wireless communication network is initialized.

The access point internal monitoring information determined by theaccess points comprises, for example, a retransmit rate to associatedsubscriber terminals, back-off windows, and/or a net allocation vectorfor the access point. By means of this, external interference statisticscan be considered in the enhanced access point related load basedroaming analysis which in turn improves decisions for roaming and loadbalancing in the wireless communication network.

Similarly to the access point related load based roaming analysisdefined above, in the enhanced access point related load based roaminganalysis processing, parameters used therein and derived from theroaming support information and the selected access point internalmonitoring information may be differently weighted. Thus, operatorspecific settings for roaming and load balancing are further improved.

Preferred embodiments of the invention are described herein below indetail by way of example with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of a WLAN network according to a firstembodiment of the present invention.

FIG. 2 shows a block circuit diagram of a load balancing systemaccording to the first embodiment.

FIG. 3 shows a flowchart of a load balancing method according to thefirst embodiment.

FIG. 4 shows a schematic diagram of a WLAN network according to a secondembodiment of the present invention.

FIG. 5 shows a block circuit diagram of a load balancing systemaccording to the second embodiment.

FIGS. 6 and 7 shows a flowchart of a load balancing method according tothe second embodiment.

DESCRIPTION OF PREFERRED EMBODIMENTS

A first embodiment of the present invention is described with referenceto FIGS. 1, 2 and 3.

Referring to FIG. 1, a wireless communication network such as a WLANcomprises several access points AP1, AP2, AP3 as communication controlelements. Furthermore, in the example shown, a backbone network isprovided which serves as a distribution network for connecting the APsto one another and to external destination points such as other WLANs orfixed networks. For connecting the APs to the backbone network, commonlyknown input/output (I/O) interfaces are used.

Each of the APs defines a cell of a specific size (indicated by thecircles surrounding the APs). Subscriber terminals T1, T2, T2 within thenetwork may be associated (connected) with one AP (serving AP) in whosecell they are located. In the present example, as the startingsituation, subscriber terminal T1 is associated with AP1, T2 isassociated with AP2, and T3 is associated with AP3.

Irrespective of its specific type (e.g. personal computer, mobile phoneand the like) a subscriber terminal comprises several means (not shown)which are required for its communication functionality and which areknown to those skilled in the art. Such means are for example aprocessor for executing instructions and processing data for thecommunication connection (e.g. transmission content and signalingrelated data), memory means for storing instructions and data, forserving as a work area of the processor and the like (e.g. ROM, RAM,EEPROM, and the like), input means for inputting data and instructionsby software (e.g. floppy disk, CD-ROM, EEPROM, and the like), userinterface means for providing monitor and manipulation possibilities toa user (e.g. a screen, a keyboard, a microphone and headset forcommunication, and the like), and network interface means forestablishing a communication connection under the control of theprocessor (e.g. and wireless interface means, an antenna, and the like).These means can be integrated within one device (e.g. in case of amobile telephone) or in several devices forming the subscriber terminal(e.g. in case of a personal computer).

Similarly, an access point comprises several means (not shown) which arerequired for its communication functionality and which are known tothose skilled in the art. Such means are for example a processor forexecuting instructions and processing data for the communicationconnection (e.g. transmission forwarding and signaling related data),memory means for storing instructions and data, for serving as a workarea of the processor and the like (e.g. ROM, RAM, EEPROM, and thelike), input means for inputting data and instructions by software (e.g.floppy disk, CD-ROM, EEPROM, and the like), user interface means forproviding monitor and manipulation possibilities to a user (e.g. ascreen, a keyboard, and the like), network interface means forestablishing a communication connection with subscriber terminals underthe control of the processor (e.g. and wireless interface means, anantenna, and the like), distribution network interface means forcommunicating via the backbone network with other APs under the controlof the processor, and the like. Besides a dedicated communicationconnection with an associated subscriber terminal, the AP is adaptedto,transmit signaling data within its cell, which enables to determineconnection quality information related to this AP. This may be performedby means of a beacon frame, which is sent permanently or in specificintervals.

In the WLAN depicted in FIG. 1, a subscriber terminal receives thissignaling data from those APs in whose cells it is located. This means,T1 receives signaling data from AP1, AP2, AP3, T2 receives signalingdata from AP1, AP2, and T3 receives signaling data from AP1, AP3. On theother hand, the subscriber terminal sends data to its respective servingAP. The signaling data may be used to determine the connection qualitysituation for the subscriber terminal, i.e. whether there is an AP otherthan the current serving AP, which provides a better communicationsituation. In such a case, the subscriber terminal initializes a roamingprocedure in order to associate with the other AP, as known in the priorart and described, for example, in connection with the respectivestandards of wireless communication networks.

Additional to this “normal” roaming procedure, according to the presentembodiment, there is provided a further functionality which is adaptedto initialize a roaming procedure on the basis of a load situation inthe WLAN, i.e. to perform a load balancing. For this purpose, thesignaling data comprises load information determined in the APs, whichis associated with an identification element identifying the sending AP.

Referring to FIGS. 2 and 3, a roaming based on a load balancingfunctionality is described in further detail.

In FIG. 2, a block circuit diagram of a load balancing system in theWLAN according to the first embodiment is shown. In FIG. 3, a flow chartillustrating a method for load balancing in a wireless communicationnetwork is illustrated.

For the sake of simplicity, in FIG. 2, only one subscriber terminal T1is illustrated for which load balancing, i.e. a roaming decision, is tobe made. However, the procedure described below is applicable for aplurality of subscriber terminals in parallel.

According to FIG. 2, the APs comprise an access point load statusmonitoring means (APLSM) 11, 12, 13 which is adapted to determine atraffic load in the respective AP1, AP2, and AP3. Furthermore, a loadcontrol device (LC) 21, 22, 23 is comprised in the APs which is adaptedto perform a load based roaming analysis which is related to accesspoint specific information and to initialize a roaming procedure of thesubscriber terminal. The load control device 21 is activated for asubscriber terminal T1, which is associated with the AP1, i.e. for whichthe AP is the serving AP (indicated by the dotted boxes of LC 22 and23). On the other hand, the subscriber terminal T1 comprises a roamingsupport means 30, which is adapted to process data and generateinformation, which are used in connection with the load balancing.

The traffic load situation may be determined in the AP as follows. TheAP knows the number of clients associated, for example, by checking alist of identification elements stored for associated subscriberterminals, e.g. corresponding MAC addresses thereof. Additionally,simultaneous traffic of the associated subscriber terminals ismonitored. The traffic-based estimation could have also some timingwindow which is observed over certain period of transmit samples. Thisis used as an indicator for the traffic load, e.g. in the form “transmitsamples=traffic load”. This could be averaged and used as a parameterfor the load information.

The load information is transmitted by the APs, for example, in form ofan AP status information (APST), which may be included in the beaconframe sent by the APs. This means that all subscriber terminals, whichare in range of the APs can receive this load information by theregularly transmitted signaling information from the APs. The APSTfurther includes an information element (e.g. MAC address) of thesending AP in order to enable an allocation of the load information tothe corresponding AP. Furthermore, in case of, e.g., a 802.11h WLAN,also dynamic frequency selection (DFS) and transmit power control (TPC)information may be included in the signaling information. (APST) fromthe APs.

The subscriber terminal may perform measurements of the communicationconnection quality during a silent period. In the present example, theroaming support means 30 of the subscriber terminal T1 receives APSTfrom AP1, AP2, and AP3 via the wireless communication interface (stepS10 in FIG. 3). The roaming support means 30 performs a processing ofthe received information in order to generate roaming supportinformation (RSUP, step S20 in FIG. 3). In this processing, for example,a statistic (or table or list) of the load situation of the available(i.e. received). APs is formed. Furthermore, for all available APs, areceived signal strength indicator (RSSI) measurement, which is commonlyknown for those skilled in the art, is executed in the subscriberterminal, e.g. by means of its processor. The RSSI measurements and loadinformation determination are allocated to the received APs by means ofthe MAC addresses of the APs and statistics (tables or lists) thereofare formed.

Additionally, a carrier to interference ratio (C/I) for thecommunication connection between each client (i.e. the subscriberterminal T1) and the available APs (i.e. per each cell) may becalculated by the subscriber terminal T1 on the basis of the receivedsignaling information. The C/I can be used as an indicator for theconnection quality with a specific AP.

As an optional further information, a terminal transmit power status canbe determined on the subscriber terminal side. This is useful in thecase of a static power control. However, in IEEE 802.11h based networks,an AP controlled transmit power for the subscriber terminals is used,i.e. where transmit power step status is available.

Moreover, the APST may comprise load information indicating the loadstatus of the subscriber terminal itself. This means, the subscriberterminal measures, e.g. in a predetermined period of time, the amount oftransmitted data and forms a corresponding statistic.

The roaming support means 30 may collect and determine these information(APST) for a predetermined period of time and form the roaming supportinformation RSUP on the basis of these collected information. For thispurpose, the information and processing results can be stored in acorresponding memory of the subscriber terminal. However, also othercriteria for stopping the formation of the RSUP are possible, forexample an instruction from the user, an indication from the serving APor the like.

When the formation of the RSUP is finished (e.g. after the predeterminedperiod of time), the roaming support means 30 sends the RSUP via theinterface means of the subscriber terminal T1 to the serving AP1, i.e.to the load control device 21 of the serving AP1 (step S30 in FIG. 3).The load control device 21 processes the received RSUP in order toperform an AP related load based roaming analysis (step S40 in FIG. 3).

In detail, the load control device 21 determines from the RSSI listincluded in the RSUP the APs, which are suitable to be a candidate for aroaming of the subscriber terminal T1, i.e. which APs provide asufficient signal strength. For example, the RSSI has to reach apredetermined threshold. From the load information related to theavailable APs, the load control device 21 can directly determine whichcapacity the respective available APs have. The load control device maycomprise a dynamically adjustable threshold value for each AP of theWLAN which indicates an “overload” situation. This means, it isdetermined how many simultaneous users are allowed and how low athroughput rate for the AP may be decreased. When a transmit powerstatus is included in the RSUP, it is possible to determine whether theavailable APs, i.e. the cells thereof, are “equal” cells. This means, itis determined whether, the subscriber terminal transmit power is insuitable level for the respective AP to receive the transmission. It isto be noted that the transmit power may differ from cell to cell,depending on the network planning. Furthermore, the C/I statisticsincluded in the RSUP can be used to determine signaling quality for thecells.

All information included in the RSUP is related to the APs by means ofthe information elements (MAC addresses). Thus, it is possible toexactly determine the communication and load situations for each of theAP1, AP2, and AP3.

The AP related load based roaming analysis executed in the load controldevice 21 uses, for example, a hand-off algorithm whose parameters arederived from the RSUP information (i.e., for example, RSSI, C/I, loadinformation, transmit power status). The processor of the (serving) APmay be used for the calculation of the hand-off algorithm. The structureof such a hand-off algorithm is manufacturer dependent and depends, forexample, on the network structure, settings for minimal signalingrequirements and the like. The hand-off algorithm used for the APrelated load based roaming analysis according to the present example issimilar to those conventionally used and known for those skilled in theart. It is to be noted that existing and even future hand-off algorithmscan be easily adapted by those skilled in the art so that the concept ofthe proposed load balancing functionality (i.e. the AP related loadbased roaming analysis) can be implemented. It is obvious for a skilledperson how to use the derived parameters within these manufacturerspecific hand-off algorithms so that the desired result is obtained.

Furthermore, it is possible that the parameters derived from the RSUPinformation are differently weighted. This means that the differentstatus information, such as RSSI, C/I, load information and the like,can be prioritized. For example, the parameter related to the loadinformation is weighted higher (has a higher priority) than theparameter related to the C/I, in particular when it is decided that anAP is “overloaded”. Thus, it is possible to use a “backdoor” in theroaming decision, i.e. to force a roaming to an AP even if the signalquality is not sufficient in normal cases. Additionally, the weightingcan be set flexibly, for example on the basis of the overall trafficsituation. For example, when the load situation of the AP is critical(traffic load is above a predetermined threshold), the parameter relatedto the load information is provided with a higher priority by the loadcontrol device 21 than in cases with a lower traffic load. Thus, it ispossible to flexibly set the criteria for the decision for a roaming.

It is to be noted that the above described weightings are only anexample and adjustable by an operator according to his/her choices.

Alternatively, for calculating the hand-off algorithm, the load controldevice 21 may use load information directly received from the serving APload information. Thus, the most current load information can be used atleast for the serving AP in the AP related load based roaming analysis.

On the basis of the results of the hand-off algorithm, the load controldevice 21 decides whether a roaming procedure is to be initialized forthe subscriber terminal T1 (step S50 in FIG. 3) This means, the loadcontrol device determines whether an AP is available for the subscriberterminal T1 which provides sufficient communication connection qualityand has less traffic load than the present serving AP1. If this is notthe case (NO), the serving AP1 is maintained and the load balancingprocedure is repeated.

On the other hand, when the load control device 21 decides that there isa “better” AP, a roaming procedure is initialized. For this purpose, theload control device 21 determines the target AP, which results from theAP related load based roaming analysis on the basis of the MAC addressincluded in the RSUP. Then, an instruction (ROAMING) indicating that aroaming to the determined new AP (for example AP2) is to be performed issent via the serving AP1 to the subscriber terminal T1 (step S60 in FIG.3). When receiving this roaming instruction, the subscriber terminal T1executes the roaming to the indicated AP2 in the known manner (step S70in FIG. 3). Thereafter, the load balancing procedure is repeated inconnection with the load control device 22 of AP2 as the new serving AP.

Next, as an exemplary situation, a load balancing procedure according tothe first embodiment is described with reference to FIG. 1.

Subscriber terminals T2 and T3 receive signaling information from twoAPs while subscriber terminal T1 receives signaling information from all3 APs. The subscriber terminals send their RSUP to their respectiveserving AP. The load control device of the respective AP processes thisRSUP and decides for a roaming of the subscriber terminal in acentralized manner. When, for example, C/I statistics are used thefollowing situation may occur. When it is determined that AP1 is“overloaded”. T1 measures the AP1, AP2 and AP3 and forms a statisticaldatabase of measurements in form of the RSUP. Since AP1 is overloaded,the analysis of the measurement shows that AP2 is less loaded. Thus, asa preliminary result, AP2 could be used as a target AP for the roaming.However, in this example, AP2 has too close channels, i.e. it is notclean. The C/I statistics shows that neighbor C/I criteria is notfulfilled. On the other hand, the analysis shows that AP3 is alsooverloaded as well. Then the selection for the roaming is to use an“unclean” channel in AP2.

The load balancing functionality can be implemented, for example, bysoftware code potions, which are loaded into the respective networkelements (subscriber terminal, access point) by means of its readingmeans and its memories. The access points are able to receive theadditional information RSUP from the subscriber terminals and to forwardit to the load control device.

Next, a second embodiment will be described with reference to FIGS. 4 to7.

It is to be noted that some of the means, functionality and proceduresof the second embodiment are parallel to corresponding means,functionality and procedures of the first embodiment. Thus, a repetitionof a detailed description thereof is omitted.

FIG. 4 shows a wireless communication network such as a WLAN comprisingseveral access points AP1, AP2, AP3 as communication control elements, abackbone network as a distribution network for connecting the APs to oneanother and to external destination points such as other WLANs or fixednetworks. Furthermore, a separate network element 100 is provided whichcomprises a load control device (LC) 110. The separate network element100 is connected to the access points by the backbone network forexchanging data.

Subscriber terminals T1, T2, T2 located in cells defined by the accesspoints AP1, AP2, AP3 may be associated (connected) with one AP (servingAP). In the present example, as the starting situation, subscriberterminal T1 is associated with AP1, T2 is associated with AP2, and T3 isassociated with AP3.

The basic elements of the subscriber terminals and the access points aresimilar to those described in the first embodiment. The separate networkelement 100 may have a similar structure like an access point, andcomprise (not shown) at least a processor, memory means, input means,reading means, interface means and the like.

In the WLAN depicted in FIG. 4, a subscriber terminal receives signalingdata from those APs in whose cells it is located. This means, T1receives signaling data from AP1, AP2, AP3, T2 receives signaling datafrom AP1, AP2, and T3 receives signaling data from AP1, AP3. Thesignaling data may be used to determine the connection quality situationfor the subscriber terminal, i.e. whether there is an AP other than thecurrent serving AP, which provides a better communication situation. Thesignaling data comprises load information determined in the APs, whichis associated with an identification element identifying the sending AP.

On the other hand, the subscriber terminal sends data based on thereceived signaling data to its respective serving AP. The AP forwardsthis data to the network element 100. Furthermore, the APs are adaptedto send separate monitoring information to the network element 100.

The network element 100 processes the forwarded data from the subscriberterminal and may send a processing result to the subscriber terminal viathe backbone network and the serving AP.

Referring to FIGS. 5 to 7, a roaming based on a load balancingfunctionality is described in further detail.

In FIG. 5, a block circuit diagram of a load balancing system in theWLAN according to the second embodiment is shown. In FIGS. 6 and 7, aflow chart illustrating a method for load balancing in a wirelesscommunication network is illustrated.

For the sake of simplicity, in FIG. 5, only one subscriber terminal T1is illustrated for which load balancing, i.e. a roaming decision, is tobe made. However, the procedure described below is applicable for aplurality of subscriber terminals in parallel.

According to FIG. 5, the APs comprise an access point load statusmonitoring means (APLSM) 111, 112, 113, which is adapted to determine atraffic load in the respective AP1, AP2, and AP3. Furthermore, theaccess points comprise an access point internal monitoring means (APIM)211, 212, 213, which is adapted to measure an external interference, toform a statistic thereof and to sent the statistic, for example, in theform of access point internal monitoring information (APIM) directly tothe load control device 110. The external interference of the AP may becaused by another AP in the network, by another non-WLAN system, such asa Bluetooth client/AP or the like. This external interference may bedetermined, for example, on the basis of measurements of retransmitrates of the AP, monitoring results of back-off windows for eachsubscriber terminal associated with the respective AP, determination ofnet allocation vector (NAV) lists for possible hidden subscriberterminals, and the like. This means, the APIM includes informationconcerning an external interference of the AP itself which can not bedetermined by measurements in the subscriber terminal T1. Of course, theAPIM can be identified to belong to a specific AP, e.g. by including theMAC address of the AP.

On the other hand, the subscriber terminal T1 comprises a roamingsupport means 110, which is adapted to process data and generateinformation which are used in connection with the load balancing.

The network element 100 comprises the load control device 110 which isadapted to perform an enhanced load based roaming analysis which isrelated to access point specific information and to initialize a roamingprocedure of the subscriber terminal. The load control device 110located in the separate network element 100 is in charge of all the APsconnected thereto and supporting the load balancing (in the presentexample AP1, AP2, AP3).

The traffic load situation may be determined in the AP by the APLSM 111,112, 113 as described in connection with the first embodiment and sentto the roaming support means 300 of the subscriber terminal T1 in rangeby means of load information included in access point status information(APST). The APST further includes an information element (e.g. MACaddress) of the sending AP in order to enable an allocation of the loadinformation to the corresponding AP.

Furthermore, in case of,. e.g., a 802.11h WLAN, also dynamic frequencyselection (DFS) and transmit power control (TPC) information may beincluded in the signaling information (APST) from the APs.

The subscriber terminal T1 may perform measurements of the communicationconnection quality during a silent period. In the present example, theroaming support means 300 of the subscriber terminal T1 receives APSTfrom AP1, AP2, and AP3 (step S110 in FIG. 6). The roaming support means300 performs a processing of the received information in order togenerate roaming support information (step S120 in FIG. 6). In thisprocessing, for example, a statistic (or list) of the load situation ofthe available (i.e. received) APs, a received signal strength indicator(RSSI) measurement ad the like is formed. The RSSI measurements and loadinformation determination are allocated to the received APs by means ofthe MAC addresses of the APs.

Additionally, statistics related to a carrier to interference ratio(C/I), a terminal transmit power status, load information indicating theload status of the subscriber terminal itself may be formed in theroaming support means 300.

The roaming support means 300 may collect and determine theseinformation for a predetermined period of time and form the RSUP on thebasis of these collected information. For this purpose, the informationand processing results can be stored in a corresponding memory of thesubscriber terminal. However, also other criteria for stopping theformation of the RSUP are possible, for example an instruction from theuser, an indication from the serving AP or the like.

When the formation of the RSUP is finished (e.g. after the predeterminedperiod of time), the roaming support means 300 sends the RSUP via theserving AP1 and the backbone network to the load control device 110 ofthe network element 100 (step S130 in FIG. 6). It is to be noted, thatfor the sake of simplicity, in FIG. 5, data exchange between thesubscriber terminal T1 and the network element 100 is shown to bedirect. However, in practice, transmission there between is performedvia the serving AP of the subscriber terminal and the backbone network.

Thus, steps S110, S120, and S130 are similar to steps S10, S20 and S30of the first embodiment except that the information are sent to the loadcontrol device 110 in the separate network element 100.

In parallel to the APST and RSUP related measures, the access pointinternal monitoring means 211, 212, 213 of AP1, AP2, and AP3,respectively, determine the information related to the externalinterference and send corresponding APIM to the load control device 110(step S140 in FIG. 6). Preferably, the determination and transmission ofthe APIM in the APs is executed in correspondence (synchronously) withthe determination and transmission of the RSUP in the subscriberterminals, i.e., for example, in predetermined time periods.

Next, the load control device 110 determines and selects those APIM,which are sent from APs being available for the subscriber terminal T1.This can be achieved, for example, by comparing the MAC addressesincluded in the RSUP and the APIM so that only APIM of those APs areconsidered for a further processing which are also received by thesubscriber terminal T1 (step S150 in FIG. 6).

The load control device 110 now processes the RSUP from the subscriberterminal T1 and the selected APIM from the access points AP1, AP2, AP3in order to perform an enhanced AP related load based roaming analysis(step S160 in FIG. 6).

In detail, the load control device 110 determines from the RSSI listincluded in the RSUP the APs, which are suitable to be a candidate for aroaming of the subscriber terminal T1, i.e. which APs provide asufficient signal strength. For example, the RSSI has to reach apredetermined threshold. From the load information related to theavailable APs, the load control device 110 can directly determine whichcapacity the respective available APs have. The load control device maycomprise a dynamically adjustable threshold value for each AP of theWLAN, which indicates an “overload” situation. This “overload thresholdvalue may be transmitted directly from the APs, e.g. by means of theAPIM. When a transmit power status is included in the RSUP, it ispossible to determine whether the available APs, i.e. the cells thereof,are “equal” cells. Furthermore, the C/I statistics included in the RSUPcan be used to determine signaling quality for the cells. Allinformation included in the RSUP are related to the APs by means of theinformation elements (MAC addresses). Thus, it is possible to exactlydetermine the communication and load situations for each of the AP1,AP2, and AP3. Additionally, the load control device 110 determines fromthe APIM whether an AP is subjected to an external interference, whichmay influence the connection quality to a subscriber terminal.

The enhanced AP related load based roaming analysis executed in the loadcontrol device 110 uses, for example, an adapted hand-off algorithmwhose parameters are derived from the RSUP information (i.e., forexample, RSSI, C/I, load information, transmit power status) and theAPIM information (i.e., for example, retransmit rate, back-off window,and the like). The processor of the (serving) AP may be used for thecalculation of the adapted hand-off algorithm. The structure of such ahand-off algorithm is manufacturer dependent and depends, for example,on the network structure, settings for minimal signaling requirementsand the like. The hand-off algorithm used for the enhanced AP relatedload based roaming analysis according to the present example is similarto those conventionally used and known for those skilled in the art. Itis to be noted that existing and even future hand-off algorithms can beeasily adapted by those skilled in the art so that the concept of theproposed load balancing functionality (i.e. the enhanced AP related loadbased roaming analysis) can be implemented. It is obvious for a skilledperson how to use the derived parameters within these manufacturerspecific hand-off algorithms so that the desired result is obtained.

Furthermore, it is possible that the parameters derived from the RSUPinformation and the APIM information is differently weighted. This meansthat the different status information and/or interference information,such as RSSI, C/I, load information, retransmit rates and the like, canbe prioritized. For example, the parameter related to the loadinformation is weighted higher (has a higher priority) than theparameter related to the C/I, in particular when it is decided that anAP is “overloaded”. On the other hand, parameters related to an externalinterference are weighted higher than parameters related to a receivedsignal strength. Thus, it is possible to use a “backdoor” in the roamingdecision, i.e. to force a roaming to an AP even if the signal quality isnot sufficient in normal cases. Additionally, the weighting can be setflexibly, for example on the basis of the overall traffic situation. Forexample, when the load situation of the AP is critical (traffic load isabove a predetermined threshold), the parameter related to the loadinformation is provided with a higher priority by the load controldevice 110 than in cases with a lower traffic load. Thus, it is possibleto flexibly set the criteria for the decision for a roaming.

It is to be noted that the above described weightings are only anexample and adjustable by an operator according to his/her choices.

On the basis of the results of the hand-off algorithm, the load controldevice 110 decides whether a roaming procedure is to be initialized forthe subscriber terminal T1 (step S170 in FIG. 7) This means, the loadcontrol device 110 determines whether an AP is available for thesubscriber terminal T1 which provides sufficient communicationconnection quality and has less traffic load than the present servingAP1. If this is not the case (NO), the serving AP1 is maintained and theload balancing procedure is repeated.

On the other hand, when the load control device 110 decides that thereis a “better” AP, a roaming procedure is initialized. For this purpose,the load control device 110 determines the target AP, which results fromthe enhanced AP related load based roaming analysis on the basis of theMAC address included in the RSUP and the APIM. Then, an instruction(ROAMING) indicating that a roaming to the determined new AP (forexample AP2) is to be performed is sent via the serving AP1 to thesubscriber terminal T1 (step S180 in FIG. 7). When receiving thisroaming instruction, the subscriber terminal T1 executes the roaming tothe indicated AP2 in the known manner (step S190 in FIG. 7). Thereafter,the load balancing procedure is repeated in connection with the newserving AP2 and the load control device 110 of the network element 100.

Next, as an exemplary situation, a load balancing procedure according tothe first embodiment is described with reference to FIG. 4.

Subscriber terminals T2 and T3 receive signaling information from twoAPs while subscriber terminal T1 receives signaling information from all3 APs. The subscriber terminals send their RSUP to their respectiveserving AP. The serving APs forward the RSUP to the load control device110 in the separate network element 100. Additionally, the APs send theAPIM to the load control device 110. When a roaming decision forsubscriber terminal T1 is to be made, the load control device 110selects all received APIM for the further processing. The load controldevice 110 processes the RSUP and APIM and decides for a roaming of thesubscriber terminal in a centralized manner. For example, there might bea situation that it is determined that AP1 is “overloaded”, and theenhanced analysis of the measurement shows that AP2 and AP3 are lessloaded. Both AP2 and AP3 have a sufficient C/I. Further, AP2 has abetter RSSI statistic than AP3. However, AP2 is subjected to externalinterference, which is derived from the APIM. Thus, the load controldevice 110 decides that the subscriber terminal T1 has to perform aroaming to AP3. The load balancing functionality can be implemented, forexample, by software code potions, which are loaded into the respectivenetwork elements (subscriber terminal, access point, network element) bymeans of its reading means and its memories. The access points are ableto receive the additional information RSUP from the subscriber terminalsand to forward it to the load control device.

Since the load control device is implemented in a fixed network element(AP or separate network element) there can be used more sophisticatedprocessing measures (e.g. hand-off algorithms) and greater storingcapacities in comparison to the rather restricted capabilities of a(mobile) subscriber terminal.

It is to be noted that features of one of the embodiments describedabove are also adaptable in the other embodiment. For example, the loadcontrol device in the first embodiment may be located in a separatenetwork element connected to the APs by the backbone network.Furthermore, in the second embodiment, the load control device may belocated in each of the APs. Then, the APs are adapted to receive theAPIM from the other APs.

Moreover, even though the above description is related to a WLAN, thepresent invention is also applicable to other wireless network types,such as mobile telecommunication networks and the like.

The load balancing functionality can be implemented in existing systemseven if there are network elements (subscriber terminals, accesspoints), which do not support the load balancing functionality. In sucha case, as for example described in the IEEE 802.11 standard,information concerning the load balancing can be ignored by thosenetwork elements which do not understand this information, and a normalroaming procedure not based on load information is performed.

As described above, by using a load balancing functionality in an accesspoint (i.e. serving access point) or in a separate network elementconnected to the access points of the wireless communication network,measurements of a subscriber terminal are processed in a centralizedmanner. Thus, the load balancing functionality can consider loadinformation from a plurality of access points and also from a pluralityof subscriber terminals. This means that an improved load balancing ispossible since the decision whether the subscriber terminal has to roamis based on an extended set of information. For example, the loadbalancing functionality can consider “future” events, when anothersubscriber terminal will change the AP, so that more capacity in thepresent AP will be available. By having the decision making in the APside load control device or in the separate network element side loadcontrol device that monitors the whole wireless communication network,the undesired ping-pong effect can be decreased. The load controlling isexecuted in a more “timing” based manner i.e. there would be slightlylonger analyzing periods over the hand-off (roaming) decisions. Alsothis represents a support in situations in which subscriber terminalsdoes not have a possibility for sophisticated measurements and hand-offalgorithms. Hence, the traffic load over the whole wirelesscommunication network is equalized.

As described above, for load balancing in a wireless communicationnetwork comprising at least one subscriber terminal T1, T2, T2 and aplurality of access points AP1, AP2, AP3, a load control device 21, 110is used which is located outside of said subscriber terminal, whereinsaid load control device is adapted to process information related to aload in said wireless communication network and to instruct roaming ofsaid subscriber terminal from an associated access point to another oneof said plurality of access points. Access point status information APSTdetermined in said plurality of access points S10, S110 is received andcommunication status information related to said plurality of accesspoints S20, S120 is determined. The subscriber terminal processes S20,S120 these information into roaming support information RSUP, which arein turn processed S40, S160 in said load control device an access pointrelated load based roaming analysis. On this basis, it is decided S50,S170 by the load control device, whether said subscriber terminal is tobe associated with another one of said plurality of access points.

It should be understood that the above description and accompanyingfigures are merely intended to illustrate the present invention by wayof example only. The preferred embodiments of the present invention maythus vary within the scope of the attached claims.

1. A method, comprising: receiving, in a subscriber terminal, accesspoint status information determined in a plurality of access points;determining, in the subscriber terminal, communication statusinformation related to said plurality of access points; processing, inthe subscriber terminal, said received access point status informationand said communication status information in order to obtain roamingsupport information; and sending, from the subscriber terminal, saidobtained roaming support information to a load control device, the loadcontrol device being located externally to the subscriber terminal. 2.The method according to claim 1, wherein the access point statusinformation comprises an access point identification element and anaccess point load status indicator determined in a respective accesspoint.
 3. The method according to claim 1, further comprisingdetermining the communication status information on the basis of atleast one of a received signal strength indicator indicating thereceived signal strength of said plurality of access points, a carrierto interference ratio per each access point, and a terminal transmitpower status.
 4. The method according to claim 1, wherein said roamingsupport information, obtained in the processing of the received accesspoint status information and the communication status informationcomprises statistics of access point related communication status andload information derived from said received access point statusinformation.
 5. An apparatus, comprising: a subscriber terminal portion;a receiver configured to receive access point status informationdetermined in a plurality of access points; a determiner configured todetermine communication status information related to the plurality ofaccess points; a processor configured to process the received accesspoint status information and the communication status information inorder to obtain roaming support information; and a transmitterconfigured to send the obtained roaming support information to a loadcontrol device, the load control device being located externally to thesubscriber terminal.
 6. The apparatus according to claim 5, wherein theaccess point status information comprises an access point identificationelement and an access point load status indicator determined in arespective access point.
 7. The apparatus according to claim 5, whereinsaid determiner is configured to determine at least one of a receivedsignal strength indicator indicating the received signal strength ofsaid plurality of access points, a carrier to interference ratio pereach access point, and a terminal transmit power status, as saidcommunication status information.
 8. The apparatus according to claim 5,wherein said roaming support information obtained by said processor fromsaid received access point status information and said communicationstatus information comprises statistics of access point relatedcommunication status and load information derived from said receivedaccess point status information.
 9. A computer program product embodiedon a computer readable medium, the computer program product beingconfigured to execute a method in a subscriber terminal, the methodcomprising: receiving access point status information determined in aplurality of access points; determining communication status informationrelated to said plurality of access points; processing said receivedaccess point status information and said communication statusinformation in order to obtain roaming support information; and sending,from the subscriber terminal, said obtained roaming support informationto a load control device, the load control device being locatedexternally to the subscriber terminal.